The very highly controlled release of the contents (core materials) of capsules is also of the greatest interest in other sectors such as, e.g., in the case of self-repairing materials, in food preservation, or in the release of pharmaceuticals or catalysts. To date, various methods for encapsulating active ingredients and the release thereof have been developed. From the pharmaceutical sector, it has long been known that capsules, after intake by the patient, are broken down in the digestive tract and thus release the active ingredients. These capsules can contain disintegrants which take up liquid in the digestive tract, and as a result swell greatly, and mechanically destroy the capsule sheath.
In addition, chemical types of release, for instance in the field of corrosion protection, are known.
External influences lead to a decomposition via, for example, a depolymerisation of the capsule sheath, and thereby to a release of the contents of the capsules. Also, photo-induced openings of capsules are described in the prior art. This includes the targeted destruction of the capsule sheath, for instance using a laser, initiating the depolymerisation of the capsule sheath, or vaporizing the contents, which leads to a disintegration of the capsule sheath. Also, electrical stimulations for the opening of capsule walls are known in the field of self-repairing electronic components and circuits. This necessitates, however, the incorporation of highly functional groups or monomers into the polymeric capsule sheath. This necessity also exists in the field of the magnetic opening of microcapsules by the incorporation of magnetically excitable functionalities, molecules, or particles on a nanocomponent size. Thermal openings are also known, e.g., via initiating the shrinkage of the capsule sheath, by thermal destruction, or by disintegrating the sheath via a pressure increase which is initiated by vaporizing the liquid core contents. Such a thermally triggered release of contents is used, for example, in the release of fragrances or deodorizing substances in cosmetics.
A disadvantage in all these opening mechanisms, however, is that they always require close matching of capsule characteristics such as wall thickness, crosslinking density, permeability, chemical composition, mechanical properties, capsule size, capsule surroundings and capsule contents. In addition, these capsule systems do not correspond to the stable core/shell systems used in industry which protect their core materials from escaping over long periods owing to their particular tightness and chemical resistance. In the case of opening mechanisms such as chemical or electrical opening, opening by light stimuli and by chemical stimuli, use is made, i.e., of the change of conformations in azo dyes, the cleavage of disulphide bonds or acetates, depolymerisation of the capsule wall by cleavage of carbamates or lipid bridges by enzymes or pH changes. This necessitates, however, the incorporation of functions into the wall material which is thereby adversely affected, for example, with respect to its crosslinking density. A high functional density is additionally required for rapid opening. The situation is similar in thermomechanical opening via electrical or magnetic stimuli. The incorporation of metal-containing nanoparticles into the capsule wall is here known which are excited to perform oscillations by applying a magnetic field or electric field, as a result of which heat is generated and thus the capsule wall destroyed. A disadvantage here is the high energy input necessitated thereby, and also the necessity of expensive special apparatuses and equipment with which the opening of the capsules and thus the release of the contents can be effected.
The simplest methods for opening microcapsules are the purely mechanically based systems, for example, crushing or squeezing the capsules.
Opening the microcapsules by thermal stimuli is of particular interest since it is easier to control and meter and can also be employed with microcapsules which are in solution or in dispersion or are in or on living creatures.
Particles have previously been described which undergo an expansion when thermal stimuli are applied. KR-A-2005/0084965, for example, describes a thermally expandable particle comprising a polymeric sheath and a volatile content which converts to the gas phase at a temperature below the softening point of the polymer. Targeted release of contents at a defined temperature is not taught, but only the thermal induced expansion of a microcapsule.
WO-A-2010/014011 describes a particle which has a polymeric sheath and contains a disintegrant which swells with water, which disintegrant, at physiological temperatures and pHs, takes up water and, after the patient takes in the particle, the capsule opens in the digestive tract of the patient and the contents thereof can thereby be released.
W. Wang et al. (Microfluidic Preparation of Multicompartment Microcapsules for Co-Encapsulation and Controlled Release of Multiple Components, Poster abstract, 19 Oct. 2011, Minneapolis Convention Center) describes a particle having a plurality of encapsulated oil kernels and a sheath which contracts on temperature elevation, tearing and thus releasing the oil kernels. A modification of the sheath for a shrinkage on change of the pH or on supply of other external stimuli such as, for example, glucose, is also described.
To date, no microcapsule has yet been described which, within a narrow temperature range, reproducibly opens with high discharge of the contents thereof and simultaneously meets the high requirements of an industrially usable system. The requirements of such a system are, i.e., low costs, flexibility with respect to adaptation to clients wishes, applications, and materials to be employed, a scale-up capacity, a favorable cost-benefit ratio and therefore high economic efficiency, meeting stability guarantees, and general simplicity of the system.